1. Technical Field
The present disclosure relates to a fuel cell system provided with a fuel cell capable of generating an electric energy by electrochemical reaction between hydrogen and oxygen.
2. Description of the Related Art
Conventionally, a fuel cell system in which a gas purge is performed when the operation of the fuel cell is stopped is known. For example, Japanese Patent Application Laid-Open Publication No. 2008-282658 discloses a fuel cell system in which a gas purge is performed when the operation of the fuel cell is stopped such that a purge gas is supplied internally so as to eliminate residual water remained in the fuel cell.
Regarding the fuel cell, when a large amount of residual water remains in the fuel cell, it may cause degradation of a start-up process of the fuel cell when the fuel cell starts at the subsequent start cycle, or cause plumbing breakage due to residual frozen water in the fuel cell. Meanwhile, when the fuel cell lacks water in the fuel cell itself, an efficiency of the power generation may be decreased.
In this respect, in the fuel cell system according to the above-described patent document, since there is a correlation between the amount of residual water in the fuel cell and the impedance thereof, an execution time for the gas purge processing is adjusted in response to a change in the impedance of the fuel cell. Specifically, a target impedance value corresponding to most appropriate amount of water to maintain the fuel cell is set in advance. Then, the gas purge processing is started and when the impedance value reaches a target value, the gas purge processing is terminated.
However, in the fuel cell system according to the above-described patent document, the impedance of the fuel cell system (impedance of entire fuel cell system) is measured by using the output voltage and the output current of the entire fuel cell system. Therefore, it is difficult to perform the gas purge processing to control an amount of water in the fuel cell accurately.
Specifically, when the air as a purge gas is supplied to the fuel cell system in order to perform the gas purge processing, it lacks of water at an input portion of the air in the respective fuel cells. Meanwhile, residual water at the input portion of the air flows towards the downstream side of the fuel cell because of flowing purge gas. As a result, excessive water tends to remain at the output portion of the air in the respective fuel cells. In this case, since the impedance value of the entire fuel cell system is calculated as a total resistance value which is sum of a resistance at the input portion of the respective fuel cells and a resistance at the output portion of the respective fuel cells, the correlation between the amount of residual water and the impedance of the fuel cell is not satisfied (i.e., variance of correlation). In other word, when the gas purge processing is executed, even when the total impedance value of the fuel cell system is matched with a predetermined impedance value set in advance that corresponds to an ideal amount of water remaining in the fuel cell, the actual amount of water may greatly differ from the ideal amount of water.